Perhaps the most labor-intensive step in the manufacture of a conventional bathroom-type scale is the calibration of the scale. Currently, conventional scales are calibrated manually by repetitive iteration; the operator repetitively adjusts the scale at zero and at a high pound level, such as 150 or 200, until the scale measures accurately. The present invention eliminates the time consuming procedure of manual calibration by providing a scale that can be calibrated by machine.
An understanding of the improvement represented by the present invention requires an understanding of the working parts of a scale and how a scale is conventionally calibrated. Conventional scales include three major components: the bottom pan, the scale mechanism and the upper housing. The scale mechanism is disposed between the upper housing and the bottom pan and includes a lever system, a main spring and a dial or weight indicator. The dials are normally round and pivot about a central axis. Weight applied to the upper housing is transmitted through the lever system to the main spring causing it to elongate proportionally to the weight. The elongation is transmitted to the dial causing it to rotate proportionally to the weight applied to the upper housing. Reading the dial through a window in the upper housing indicates the weight.
In order to ensure acceptable accuracy of the scale, the complete scale must be calibrated so that the weight indication is correct at the applied weight and at the zero position with no weight applied to the upper housing. The two parts employed by conventional scales for calibration purposes are the adjusting nut and the calibrating stud.
A conventional calibrating process is performed as follows. As an assembled scale comes off the assembly line, an operator places the scale on a flat surface. At this point, no weight is applied to the scale. Normally the scale dial must be "zeroed" by adjusting the adjusting nut. Then, a predetermined calibration weight, such as 200 pounds, is applied to the top of the scale. Normally, the scale will not read exactly 200 pounds because the main spring of the scale must be calibrated. Main spring elongation is not properly proportioned to the weight applied thus giving an incorrect weight reading. The calibration weight is now removed from the scale. Adjustment of the main spring elongation is accomplished by rotating the calibration stud until the dial reads the predetermined calibration weight (200 pounds). The scale must be re-zeroed since the adjustment to the calibration stud will necessarily affect the zero reading of the scale. Thus, the adjusting nut is turned until the zero on the scale lines up with the read mark. After the scale is re-zeroed, the calibration weight is again applied to the scale and the operator determines whether the calibration stud must be re-adjusted. Very often the calibration weight is now slightly out of adjustment, though closer to the designated weight than before.
A scale is acceptably calibrated when two successive and accurate readings are attained, either an accurate zero reading followed by an accurate calibration weight reading, or vice versa. To acceptably calibrate a conventional scale, an operator must normally perform successive zero and calibration weight calibrations two to three times. On occasion, an operator must perform these calibrations four to five times. Thus, the calibration procedure takes time and limits the rate of manufacture.
The present invention overcomes the above problem by providing a scale that may be calibrated by a machine, rather than manually. By calibrating conventional scales by a machine, the manufacturer can eliminate a labor-intensive step in the manufacturing process and thereby reduce labor and manufacturing costs.